As a power supply of an electronic apparatus, a series regulator, a switching regulator, or a DC/DC converter is used. For example, a series regulator is structured by using a feedback amplifier having a reference voltage input terminal to which a reference voltage is input, a feedback input terminal to which an output is input via a feedback gain, and an output terminal section.
The regulator or the DC/DC converter performs feedback control that compares between the output voltage and the reference voltage by using an error amplifier so as to let the output voltage be within a predetermined range.
Also, the output of the reference voltage of the reference voltage circuit of the regulator is sharply started up when the power supply is energized. For this reason, an excessive overshoot appears in the output of the regulator. Therefore, some regulators have a soft start function that controls the output voltage during the starting-up period until the output voltage is completely started up. The soft start function controls a gradient exhibiting a voltage change in a process from a state in which the output voltage is 0 V until the output voltage is started up to a predetermined voltage, in order to reduce a rush current and to alleviate the overshoot at the time of starting-up of the output voltage of the regulator or the DC/DC converter.
Patent literature 1 discloses an overshoot control circuit for a voltage regulator that supplies a gate voltage needed to maintain a constant output voltage to an output transistor by an error amplifier that compares a reference voltage with an output voltage subjected to voltage division by a feedback resistor wherein a CR circuit is provided to an output of the aforesaid reference voltage.
Patent literature 2 discloses a power supply circuit and a power supply apparatus having a soft start control section that detects an input of a first soft start voltage, outputs a first soft start voltage when a soft start function control signal is on, and outputs a second soft start voltage having a gradient different from that of the first soft start voltage when the soft start function control signal is off.
FIG. 1 is a circuit diagram of a regulator having a soft start function.
Referring to FIG. 1, regulator 10 includes power supply voltage input terminal Vin, control voltage input terminal Vcont, connection terminal Cs for connecting externally attached capacitor C, output transistor Q1, bias circuit 11, reference voltage circuit 12, CR circuit 13, error amplifier 14, voltage-division resistors R1 and R2, output voltage Vout output terminal Vo, and output capacitor Cout.
CR circuit 13 is made of resistor R and externally attached capacitor C that are inserted between error amplifier 14 and the reference voltage. CR circuit 13 realizes the soft start by allowing the starting-up of the reference voltage at the time of energizing the power supply to have a CR time constant. CR circuit 13 constitutes a soft start circuit that alleviates the overshoot of the output voltage by gradually raising the reference voltage output and preventing a rush current that charges up output capacitor Cout when the power supply is energized. Here, capacitor C may be incorporated within regulator 10.
Output transistor Q1 is connected between power supply voltage input terminal Vin and output voltage Vout output terminal Vo, and output capacitor Cout is connected to output voltage Vout output terminal Vo. Output capacitor Cout is a stabilizing capacitance for phase correction and noise removal.
Output voltage Vout is subjected to voltage division through resistors R1 and R2, and a feedback voltage drawn out from a connection middle point thereof is connected to a non-inverting input terminal of error amplifier 14.
Error amplifier 14 is specifically made of an operation amplifier, where a feedback voltage obtained by voltage division of output voltage Vout by resistors R1 and R2 is input to the non-inverting input terminal, and the reference voltage is input to an inverting input terminal via CR circuit 13. Error amplifier 14 performs voltage feedback by supplying an output voltage that accords to a difference between the feedback voltage and the reference voltage to a gate of output transistor Q1, whereby the output voltage of regulator 10 is maintained to be a constant voltage.
In the above-described structure, regulator 10 gradually starts up the input and output of the error amplifier by the CR time constant connected to the reference voltage. This can reduce an amount of overshoot of regulator 10 output at the time of energizing the power supply.